1. Field of the Disclosure
The present invention relates generally to imaging. More specifically, examples of the present invention are related to complementary metal oxide semiconductor based image sensors.
2. Background
Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (CMOS) image sensors (CIS), has continued to advance at a great pace. For example, the demands for higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
Two fields of applications in which size and image quality are particularly important are security and automotive applications. For these applications the image sensor chip must typically provide a high quality image and have improved sensitivity in the near infrared portion of the light spectrum. In order to achieve these characteristics, the photosensitive apertures should be as large as possible and have very deep photodiode regions to collect more near infrared light.
The pixel (i.e., picture element) fill factor denotes the fraction of the surface area of a pixel that is sensitive to light. Pixel pitch is the physical distance between adjacent pixels in an imaging device. Pixel fill factor has become smaller as pixel pitch has been reduced because the active circuit elements and metal interconnects consume an increasing proportion of the area in each pixel as the photosensitive regions of pixels are reduced in size.
One way to address the loss of pixel fill factor is to use a microscale lens (i.e., microlens) directly above the photosensitive portion of each pixel to focus the light directly towards the photosensitive portion of the area within the pixel. Another way to address the loss of pixel fill factor is to use backside illuminated (BSI) image sensors, which place the active pixel circuit elements and metal interconnects on a frontside of an image sensor die and the photosensitive element within the substrate facing a backside of an image sensor die.
One of the transistors included in a pixel is commonly referred to as a transfer transistor, which includes a transfer gate disposed between the photosensitive element and the floating diffusion of a pixel. The transfer gate is disposed on a gate oxide. The photosensitive element, floating diffusion region, and gate oxide are disposed on a substrate.
During operation, a conducting channel region may be formed under the transfer gate when a bias voltage is applied to the transfer gate such that charge is transferred from the photosensitive element to the floating diffusion region of the pixel. A degraded image may result from the conventional pixel structure being unable to remove all of the charge from the photosensitive element such that a residual signal remains during successive readings of the pixel. This leftover information remaining in the photosensitive element, which degrades image quality, is often referred to as image lag, residual image, ghosting or frame to frame retention. In order to maximize the image quality derived from ever smaller pixels, much attention has been paid to improving carrier transfer out of the photodiode by optimizing the details of the transfer gate structure and its proximity to the photodiode.
In addition, maximizing the number of carriers that a photodiode can hold, which may also be referred to as full well capacity, as well as maximizing charge to voltage conversion when reading signals from pixels have been a focus as efforts to miniaturize image sensors have continued. Photodiodes of decreasing size have also been optimized to have greater spectral sensitivity to compensate for their reduced collection areas. For example, small photodiodes that are doped very deeply into the substrate have been developed to improve red and infra red sensitivity.
As pixel dimensions become smaller, the background current may become a larger fraction of the total signal, which reduces the signal to noise S/N ratio and dynamic range (DNR). For some applications, the loss of DNR and attendant image quality degradation has reversed the drive to miniaturize at least the photodiode element of an image sensor in order to increase the number of available signal carriers for applications involving low light, high contrast scenes, or long wavelength light. With increased distances between the outer reaches of the photodiode and the transfer gate there may be reduced lateral electric fields driving the signal carriers toward the transfer gate. With signal carriers then depending more on diffusion for their transfer out of the photodiode, more signal carriers may be left behind and result in image lag. For larger photodiodes, it becomes even more important to more fully extract signal carriers to reduce image lag.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.